1. Field of the Invention
The present invention relates to a multi-color image forming apparatus of the type in which a required multi-color image is obtained by transferring a plurality of color toner images having different color toner developed thereon onto a printing medium one above another. More particularly, it relates to a multi-color image forming apparatus in which a plurality of image carriers in the form of photosensitive drums or the like are disposed in parellel with one another in spaced relation and each of color toner images is successively transferred onto the printing medium after completion of latent image building and developing on each of the image carriers. The apparatus of the invention is advantageously applicable to a recording device, copying machine, printer or the like.
2. Related background art
As is hitherto known, a variety of electrophotographic multi-color image copying apparatuses have been developed and some of them are put into practical use on a commercial basis. An apparatus as shown in FIG. 6 is one of conventional copying apparatus of the above-mentioned type. As is apparent from the drawing, the apparatus includes first to fourth image forming sections Pa, Pb, Pc and Pd which are provided with photosensitive drums 1a, 1b, 1c and 1d. Latent image forming means 2a, 2b, 2c and 2d, developing means 3a, 3b, 3c and 3d, transferring means 4a, 4b, 4c and 4d and cleaning means 5a, 5b, 5c and 5d each of which are well known for any expert in the art are arranged around the photosensitive drums 1a, 1b, 1c and 1d. First, a latent image having yellow component color derived from an original image is built on the photosensitive drum 1a with the aid of the latent image forming means 2a in the first image forming section Pa by way of the steps of electrical charging and exposing and the thus formed latent image is then developed with yellow toner under the effect of the developing means 3a. Finally, the yellow toner image is transferred onto a printing medium 6 with the aid of the image transferring means 4a.
While the yellow toner image is transferred onto the printing medium 6, a latent image having magneta component color is built in the second image forming section Pb and the thus formed latent image is developed with magenta toner under the effect of the developing means 3b. When the printing medium 6 which has completed transference in the first image forming section Pa is then conveyed to the image transferring means 4b in the second image forming section Pb, a magenta toner image is transferred onto the printing medium 6 at a predetermined position where the latter has carried a yellow toner image thereon. Thereafter, image transference is carried out in the same manner as mentioned above for both cyan color and black color and after completion of image transference fixing is effected with the aid of fixing means 7 whereby a multi-color image is obtained on the printing medium 6. On completion of image transference in that way each of the photosensitive drums 1a, 1b, 1c and 1d is entirely free from residual toner with the use of the cleaning means 5a, 5b, 5c and 5d. Now, the next image building is ready to be initiated.
However, it is found with respect to the conventional apparatus as mentioned above that a problem of image deviation occurs unavoidably due to the fact that multiple image transference is achieved while the printing medium 6 is conveyed by means of a conveyor belt 8. An extent of image deviation varies widely in dependence on accuracy in movement of the conveyor belt and rotation of the photosensitive drums. To obviate the above-mentioned problem there have been already made such proposals that a driving roller having high concentricity is used to drive the conveyor belt and a train of driving gears having high dimensional accuracy is employed for the driving system.